Climate and Groundwater at Casa Grande Ruins National Monument, Water Year 2018

In the Sonoran Desert, water is the main factor limiting the lives of plants and animals, so climate and hydrology shape entire ecosystems. At Casa Grande Ruins National Monument in Arizona, the National Park Service's Sonoran Desert Network monitors climate and groundwater, which are closely linked and essential for understanding the condition of the monument's plants, animals and cultural resources.

An arid monument

The monument's climate is classified as arid, based on an aridity index, a global measure that compares long-term average annual precipitation with average annual potential evapotranspiration — the water that would evaporate or be used by plants if water were unlimited.

On the scale from hyperarid to subhumid, Casa Grande Ruins National Monument is arid. NPS

Water year 2018

Findings are reported by water year, which runs from October through the following September, and compared with the 1981–2010 normals.

Temperature. The year was generally warmer than normal. In every cool-season month (October–March), average highs and lows were 1 to 9°F above normal. From May to September, highs were mostly near normal, and April was about 4°F warmer than normal. Lows were 1 to 5°F above normal in every warm-season month.

Precipitation. The monument received only 5.81 inches of rain, 63% of the normal 9.18 inches. The monsoon was wetter than usual, but the rest of the year was much drier: no rain fell in October–November or March–May. In June, a single 0.65-inch storm delivered 1,300% of that month's normal rainfall. The regional drought that began in 2000 continued.

Extremes. Extreme weather matters as much as averages. Prolonged cold can damage keystone plants such as saguaro cactus and velvet mesquite, and extreme rain can cause flooding and erosion, affect plant growth and change animal behavior — and, at Casa Grande, erode soil and expose archeological resources. In 2018 there were far fewer extremely cold days than normal (4, compared with 16.8), and just one extreme rain event of more than an inch (1.05 inches on September 20), compared with an average of 2.3 a year.

Weather stations at Casa Grande Ruins National Monument. Image from the National Park Service's page.

Groundwater

The monument sits in the north-central part of the Eloy groundwater subbasin, which has been in deficit since the early 20th century: more water has been pumped out than nature has replaced. The result has been changes to natural water flow, falling water levels and land subsidence — the gradual collapse, cracking or slumping of ground where deep, sediment-filled basins are drained. Groundwater supplies drinking water to the monument and neighboring Coolidge, and its loss could affect both the water supply and the stability of the monument's structures.

In water year 2018, groundwater levels near the ruins rose 12.3 feet from the year before, a departure from the general decline since 1995. From March 2017 to March 2018 the land subsided 0 to 1 centimeter across most of the monument and up to 2 centimeters in its northwest corner. Only a few minor earth fissures are known in the Coolidge area.

Sources

• National Park Service, Sonoran Desert Network, "Climate and Groundwater Monitoring at Casa Grande Ruins National Monument": https://www.nps.gov/articles/sodn_cagr_h2018.htm
• The page summarizes K. Raymond, Status of climate and water resources at Casa Grande Ruins National Monument: Water year 2018 (2019). A chart made with a non-federal online tool and a map credited to the Arizona Department of Water Resources are not reproduced.